1. Field of the Invention
This invention relates to a composite photosensitive element. It particularly relates to a photosensitive element which can be stored in roll form without the need for a coversheet and is useful as a photoresist.
2. Description of the Related Art
Photosensitive compositions are well known and are useful as photoresists in the formation of printed circuits, in the formation of lithographic printing plates, and in proofing applications. In such systems, actinic radiation impinges on a material containing at least one photoactive component to induce a physical or chemical change in that material. The latent image thereby produced can then be processed to form a patterned mask or an image. The photosensitive systems can be positive-working or negative-working. In positive-working systems, the areas exposed to actinic radiation are removed in the post-exposure processing step; in negative-working systems, the areas not exposed to actinic radiation are removed. Particularly useful compositions are negative-working photopolymerizable and/or photocrosslinkable compositions, hereinafter referred to collectively as "photopolymerizable". In such systems, exposure to actinic radiation initiates polymerization and/or crosslinking reactions, resulting in insolubilization of the material in suitable developer solutions. The latent image is developed by treating with the developer solution. The photopolymerizable compositions generally contain a binder, at least one monomeric or oligomeric material capable of polymerizing and/or cross-linking, and a photoinitiator.
The photopolymerizable compositions are generally formed as a continuous dry film on a base support. The dry film is generally sandwiched between the base support film and a coversheet and stored in roll form. As the roll is formed, a boundary layer of air is wound with the film between the laps. The surfaces of the base and coversheet are generally roughened to allow the air layer to bleed out the sides of the roll, resulting in smooth rolls.
When using the photosensitive film, it is conventionally unwound from the roll, the coversheet is removed, and the film is laminated to a substrate using a hot roll or vacuum laminator for subsequent exposure, development, etching and/or plating. Thus the coversheet is only used for protecting the film during storage and results in added cost. It would be desirable to eliminate the coversheet. However, the photosensitive compositions are frequently somewhat tacky and thus, in the roll, stick to the backside of the next lap immediately on contact. Flint et al., U.S. Pat. No. 4,293,635, disclose a photopolymerizable composition including an amphoteric interpolymer which can be stored in roll form without a coversheet, with a base support treated with a release layer on the backside to prevent the sticking of one layer to another. However, while the backside release layer facilitates unwinding of the roll without sticking, it does not allow the trapped air to bleed out. The pressure between laps of the roll and small differences in coating thickness tend to concentrate the air in large bubbles, which cause depressions in the resist layer, in turn causing defects in the circuit board. Thus it would be desirable to have a photosensitive element that could be stored in roll form without a coversheet without air entrapment between the layers of the roll.